This invention relates to hydroforming of dual wall conduit elements, and particularly to a hydroforming method and apparatus for forming dual wall, air gap conduit elements, particularly for engine exhaust system components.
Hydroforming of conduits such as engine exhaust components is known, as set forth for example in U.S. Pat. No. 5,170,557. Such components with dual walls separated as by an air gap have proven to be particularly effective in increasing efficiency of downstream exhaust catalytic converters etc., as well as controlling noise. Copending application Ser. No. 065,126, now U.S. Pat. No. 5,363,544 sets forth a hydroforming method and apparatus for creating such components in successive cavities of a mold.